The present invention relates to low noise amplifiers, and more particularly, to a high input impedance low input capacitance amplifier that is well suited for image sensors.
While imaging arrays based on CCDs have become common, these arrays have two drawbacks. First, the technology used to fabricate such arrays has a significantly lower yield than that used to fabricate CMOS circuitry. CCD arrays have large areas of gate oxide. These areas are prone to electrical shorts. These electrical shorts, in turn, reduce the yield of useful chips, and hence, increase the cost of the devices.
The second problem with CCD arrays lies in the lower bound for the noise in the sensor arrays. Many imaging problems of interest require the imaging array to sense very low levels of light. The minimum level that an array can sense depends on the minimum noise in the sensors.
In principle, both of these drawbacks can be overcome by utilizing CMOS image sensors. The CMOS yields are significantly better than those of the CCD fabrication process. In addition, the minimum noise levels achievable with CMOS-based sensors are substantially lower than those that can be obtained with CCDs.
Each pixel of an image sensor typically includes a photodiode and an amplifier for amplifying the signal from the photodiode. If the image sensors are fabricated in sufficient volume, the fabrication line can be xe2x80x9ctunedxe2x80x9d to provide amplifiers with sufficiently low 1/f noise. The cost of the tuning and the costs associated with being restricted to a particular fabrication can be amortized over the large volume of devices. However, many image sensors of interest do not command such high volumes and must be compatible with a variety of production lines.
Broadly, it is the object of the present invention to provide an improved amplifier and image sensor utilizing the same.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is an imaging element that includes a photodiode and an amplifier for providing an output signal indicative of the voltage across the photodiode. The amplifier includes an input follower, a level shifter, and an output amplifier. The input follower has a follower output having a signal related to the voltage across the photodiode. The level shifter isolates the follower output from the output amplifier input and has a first terminal connected to the follower output and a second terminal connected to the input of the output amplifier. The input follower includes an input transistor connected to the photodiode, and the output amplifier has an input transistor connected to the second terminal of the level shifter. The sizes of these transistors are chosen such that the area of the input transistor in the input follower is less than the area of the input transistor in the output amplifier. In the preferred embodiment of the present invention, the input follower utilizes PMOS transistors. The level shifter provides DC isolation between the input follower and the output amplifier. A shorting switch for shorting the input and output terminals of the output amplifier may also be included.